CONCEPTS IN BIOLOGY

PART VI. PHYSIOLOGICAL PROCESSES

25. Nutrition. Food and Diet

25.5. Determining Energy Needs

Significant energy expenditure is required for muscular activity. However, even when the body is at rest, energy is required to maintain breathing, heart rate, and other normal body functions. The basal metabolic rate (BMR) is the rate at which the body uses energy when it is at rest. The basal metabolic rate of most people requires more energy than their voluntary muscular activity. Much of this energy is used to keep the body temperature constant. A true measurement of basal metabolic rate requires a measurement of oxygen used over a specific period under controlled conditions.

Several factors affect an individual’s basal metabolic rate, including age, gender, height, weight, and fundamental differences in metabolism. Basal metabolic rates decline throughout life. Children have high basal metabolic rates and elderly people have low basal metabolic rates. In general, men have higher metabolic rates than women. The larger a person, the higher his or her metabolic rate. With all of these factors taken into account, most young adults fall into the range of 1,200 to 2,200 Calories per day for a basal metabolic rate.

Because few of us rest 24 hours a day, we normally require more than the energy needed for basal metabolism. One of these requirements is specific dynamic action (SDA), the amount of energy needed to process the food we eat. It is equal to approximately 10% of the total daily Caloric intake.

In addition to basal metabolic rate and specific dynamic action, the activity level of a person determines the number of Calories needed. This is known as voluntary muscular activity. A good general indicator of the number of Calories needed above basal metabolic rate is the type of occupation a person has (table 25.6). Because most adults are relatively sedentary, they would receive adequate amounts of energy if women consumed 2,200 Calories and men consumed 2,900 Calories per day.

TABLE 25.6. Additional Calories as Determined by Occupation

Occupation

Calories Needed per Day Above Basal Metabolic Rate*

Sedentary (student)

500-700

Light work (businessperson)

750-1,200

Moderate work (laborer)

1,250-1,500

Heavy work (professional athlete)

1,550-5,000 and up

* These are general figures and will vary from person to person, depending on the specific activities performed in the job.